How to engineer manufacturing, industrial buildings: HVAC
- C. Erik Larson, PE, LEED AP BD+C, Principal, Industrial Systems, Wood Harbinger, Bellevue, Wash.
- Ronald R. Regan, PE, Principal, Triad Consulting Engineers, Morris Plains, N.J.
- John Schlagetter, NCARB, PMP, CSI, CCS, CCCA, LEED Green Associate, Senior Architect, Process Plus, Cincinnati
- Wallace Sims, SET, NICET Fire Alarm Level IV, Lead Life Safety Engineer, CH2M Hill, Portland, Ore.
CSE: What unique HVAC requirements do manufacturing/industrial structures have that you wouldn’t encounter on other structures?
Regan: Manufacturing plants are designed based on the requirements for the products being manufactured, which in many cases are very specific. Typical requirements may be:
- High and low temperature depending on the product
- High and low humidity requirements depending on the product
- High volume of air (air changes per hour) depending on the operation
- Large quantities of outdoor air requirements.
All of these requirements are unique to specific products for the pharmaceutical industry, food industry, petrochemical plants, etc. Such stringent requirements are not needed for normal commercial structures. Unlike HVAC systems for commercial structures, which are designed based on occupant comfort and code compliance, HVAC systems for manufacturing structures are designed based on the product requirements.
Larson: There are several factors that impact the HVAC design for what I normally consider to be large, wide-open spaces. These factors include pressurization, humidity control, and dust control. With lots of different point-of-use exhaust systems, dust control can be provided, but this can have an undesired impact on pressurization. If there are processes that rely on tight humidity control, pressurization is even more important to consider. Of course, encapsulation of any area with humidity control is key—and don’t forget to manage airflow through the entrance/exit points.
Schlagetter: Higher-quality filtered air, increased control of humidity, greater requirement for once-through air versus recycled, and pressurization differences between spaces to control for airborne contaminants.
CSE: What changes in fans, variable frequency drives (VFDs), and other related equipment have you experienced?
Regan: Modern fan systems are more efficient, and by introducing the VFD to such systems we can improve the overall energy efficiency of the systems and provide much better comfort levels to occupants. VFDs in the HVAC industry are used for varying the air and water flow to the building load pumping systems. Most commercial HVAC systems incorporate variable air volume (VAV) systems. These systems are served by variable volume air handlers, supplying conditioned air through a metal duct distribution system with variable air terminals serving the spaces. The majority of these air handlers include inlet guide vanes that control the quantity of air supplied depending on the building load dictated by the individual thermostats controlling the variable air terminals serving the spaces. As these air terminals demand more or less air, the system pressure increases or decreases changing the position of the inlet guide vanes, increasing or reducing the pressure in the system. While using this method saves some energy by riding the fan curves, the more efficient and controlled way of achieving energy savings and system operation is by using a VFD. We also use VFDs in commercial buildings with water systems requiring constant pressure regardless of flow rate.
Larson: More and more flexibility and efficiency are available as mainstream vendors expand their offerings. Sustainable design has pushed fan-wall technology, and fan arrays have allowed us to design air handling systems with a much larger range of operation. This provides the flexibility we need to truly take maximum advantage of turn-down, and it allows small portions of an area served by a large air-handling unit (AHU) to be controlled in an occupied mode without wasting energy, keeping the whole space conditioned.
CSE: What indoor air quality (IAQ) challenges have you recently overcome? Describe the project, and how you solved the problem.
Schlagetter: On an NFPA 45 Class A laboratory complying with NFPA 30, we worked with the owner, facility manager, and the HVAC, controls, and process safety contractors to provide tempered once-through MERV 13-iltered low-level supplied air that is continuously exhausted through ducted grills. This was along with seven exhaust hoods, two capture hoods, and lower explosive limit (LEL) sensor-actuated purge exhaust that enters from dampered rooftop outside air intake hoods through fire-dampered pressure-relieving wall louvers to maintain an ISO Class 8 drug-grade, GMP-compliant environment.
Larson: Sustainable design drives a desire to minimize energy consumption associated with conditioning outside air. This involves a lot of instrumentation, and every instrument is a potential problem if it’s not reading whatever it’s supposed to measure with enough accuracy. This can be particularly difficult for air measuring units, which rely on adequate straight lengths upstream and downstream of the sensor. If you’re varying the airflow over a wide range, it can be a real challenge to get accurate readings over the entire range, as sometimes the sensor’s response is not as linear as it should be. Where the duct arrangement is challenging or the range of measurement is large, consider implementing a calibration curve instead of a calibration constant. Make sure you require that all outside airflow rates that are part of your HVAC controls be double-checked against a calibrated instrument. It’s OK to provide minimum ventilation air, but you need to do your part to make sure the occupants aren’t getting shorted.
Regan: Recently we were introduced to a client with an IAQ problem in a high-rise residential building managed by his company. The problem consisted of insufficient outdoor ventilation air to the indoor corridors. This issue not only created an IAQ problem, it also created a high negative pressure in the building. By not having sufficient outdoor air, the individual exhaust systems in the apartments were creating a high negative pressure in the building. The makeup air was made up by unconditioned hot or cold air introduced through any opening such as loading dock doors, pedestrian doors, or any other way the air could be introduced into the building. The large quantities of air required to overcome the problems were an issue with the existing infrastructure. A new system was designed to introduce the proper amount of outside air properly conditioned to maintain IAQ and avoid the negative pressure in the facility. The new design incorporated a heat recovery system where exhaust air from the building was used to precondition the outside air introduced to the building. The system eliminated the problems and provided the client with an energy-efficient solution.
CSE: In your experience, have alternative HVAC systems become more relevant? This may include displacement ventilation, chilled beams, etc.
Larson: Yes. Every new project has a lot more technology to choose from than in past years, and for some projects, these alternative systems do prove to be more appropriate. The key is to know enough about the building that you can be sure the HVAC system will provide the performance that it’s capable of. For example, if you don’t have simultaneous heating and cooling loads, VRFs or water source heat pump systems won’t provide the benefit that they could. Similarly, if you’re in a climate like ours, if you’re not installing air side economizers, you’re probably missing out (unless you’ve been able to implement natural ventilation). We start every project with a philosophy that nothing is off the table, then start evaluating the different options to see what can be crossed off the list and what needs further evaluation.
CSE: Do you find it more challenging to retrofit HVAC systems on older buildings than installing on new? Please explain your answer.
Regan: It is definitely more challenging to retrofit HVAC systems on older buildings than installing a new HVAC system in a new building. In a new building you start with a blank sheet of paper and the design is coordinated with the architect and other professionals to incorporate the spaces and requirements needed for the HVAC system. When you are retrofitting an existing building, you must perform an in-depth investigation of the existing conditions and space available to you for equipment rooms, risers, ceiling space, etc. The fact that it is an existing facility can create major code compliance issues. Many of the existing facilities were originally designed with much lower code compliance requirements than the present-day requirements. These requirements can affect not only the space but also the structural integrity of the facility. The existing conditions must be well identified on your design to avoid cost overruns to your client.
Schlagetter: Increased filtration can drive larger fans to overcome pressure loss, which drives equipment sizes that can be difficult to accommodate. Increased ventilation rates and IAQ can drive increased duct sizes with similar physical constraints, especially in piping-intensive environments. Increased ventilation rates and humidity control can drive the need for energy recovery and dehumidification. Also, humidification can stress piped utilities and again be challenged physically by a lack of space.
Larson: Retrofit and renovation are always more challenging because there are more constraints. There is also often a requirement to keep facilities in operation throughout the upgrades. New buildings offer more opportunities to implement new technology and to optimize sustainable design practices because you’re starting with a clean slate. Personally, I prefer the challenge of retrofit construction because the first task is to figure out what you really have to work with, and you have the added benefit of being able to see how the space is being used today. This can really improve the chances that you’re going to deliver a facility that truly meets your client’s needs.